This invention is in the field of fibrous reinforced resin sheets; more particularly, the invention relates to a method of manufacturing fibrous reinforced sheets.
Thermosetting and thermoplastic fibrous reinforced sheets and methods of making them are known in the art. For the purposes of the present invention sheets include fiberglass reinforced mats and panels having flat, corrugated or otherwise formed cross sections in the plane transverse to the longitudinal axis of the sheet being formed. Such fibrous reinforced sheets and methods of their manufacture are described in U.S. Pat. Nos. 3,071,180; 3,072,958; 3,291,878; 3,894,134; and 4,255,217.
Generally the fibrous reinforced resin sheets are made of a crosslinkable resin with crosslinkable polyester being most popular. The fibrous reinforcement can be cut glass fibers. The resin is mixed with a crosslinking agent. Cut glass fibers can be added on a carrier web. The composition is then formed into a sheet and the composition in the sheet is crosslinked. The crosslinking usually takes place at elevated temperatures.
A top film can be placed over the composition on the carrier web prior to forming into a sheet. In this way the fibrous reinforced resin material is in sheet form having the carrier web as an outside layer on one side and the top film as an outside layer on the opposite side. The sheet can then be crosslinked and pressed into a desired configuration. For example, the sheet can be corrugated along its longitudinal axis. Finally, the carrier web and the top film may or may not be removed prior to cutting the sheet to a desirable size.
A problem that has been encountered particularly in crosslinking and/or forming the fibrous reinforced resin sheet has been the maintaining of the dimensional stability of the film in the transverse dimension during heating. Films presently in use tend to shrink in the transverse direction causing the film to wrinkle. This causes wrinkles in the resin sheet and some variation in the sheet thickness. This difficulty has been particularly accentuated when dealing with liquid resins such as unsaturated polyester which are crosslinked under heat or molten thermoplastic resins which are thermoformed.